Crews from Ossett and Wakefield were called to Cathedral Academy in Wakefield at around 6am this morning.
The West Yorkshire Fire and Rescue Service investigations team said staff at the school, on Thornes Lane, closed a door to prevent the fire spreading to other parts of the building.
Watch Commander of Wakefield Fire Station Simon O’Hara added: “One of the classrooms was totally engulfed in fire.
“Fortunately, fire protection systems including an alarm and early warning system and fire doors, helped to stop the flames from spreading, as did the quick actions and intervention of fire crews.”
Mr O’Hara said the blaze was discovered by a cleaner, who was in the school with the caretaker, when the alarm was triggered.
He said both members of staff escaped unhurt.
A post on the West Yorkshire Fire and Rescue Service Twitter feed said: “Fortunately no-one was harmed in this fire, which happened shortly after 6am today. Sadly pupils’ artwork was lost.”
The Academy tweeted this morning: “Due to a small fire overnight, which is now contained, and health and safety reasons, we are closed to Year 7 to 10 students today.
“We are open as normal for Year 11, post 16 students and examinations today.”
The school will be open as normal tomorrow.
The cause of the fire is still being investigated.